Is this a symptom of a blown OT?

[Dodge]

Hey guys,

I've got an Emery Superbaby and I was doing my usual playing the amp dimed into a 1x12" Celestion V30 cab via a MiniMASS 50, and suddenly the amp volume disappeared.

It wasn't completely silent, it's now at speaking volume when fully dimed (about 10% of the usual level) and the distortion is pretty nasty and fluctuates in volume very slightly.

Have I fried the OT? All the tubes are fine (I've subbed others with the same results), resistors measure correctly but I've got no means of measuring the caps.

Thanks in advance.

Dodge

[Gtr_tech]

Could very well be....that's why we call attenuators "tranny toasters". First (discharge filter caps) check screen resistors, then check the output xfmr primary winding. If you can verify the preamp ok, and plate voltages for both front end and output section, I guess the next step would be to wire in another output xfmr and see if that cures it. The can fail by shorting turns inside and thats hard to detect. Also look for carbon tracks on *both* sides of the pwr tube socket, and check for coupling caps leaking DC.

I can't really get into specifics like voltages you should expect as I've not been inside one of those yet.

[Enzo]

How does teh amp work when the Minimass is removed? WE should consider the attenuator could have failed. Eleminate that possibility before tearing into the amp.

I don't know the amp, does it have an effects loop, headphones jack, or external speaker jacks? All of those jacks can cause such symptoms when they need cleaning.

[Gtr_tech]

Its a pretty simple amp....basically like a Champ 12 only ptp wired and way less junk in there....just single channel.
http://www.emerysound.com/Superbaby.html
I don't think there's anything other than an input and spkr jack.
I think the attenuator is overkill for a small SE amp like that anyway. Thats the whole point of the low power amp isn't it? Being able to crank it without blowing the windows out. Kids....use them tranny toasters at you own risk.....

[Dodge]

Could very well be....that's why we call attenuators "tranny toasters". First (discharge filter caps) check screen resistors, then check the output xfmr primary winding. If you can verify the preamp ok, and plate voltages for both front end and output section, I guess the next step would be to wire in another output xfmr and see if that cures it. The can fail by shorting turns inside and thats hard to detect. Also look for carbon tracks on *both* sides of the pwr tube socket, and check for coupling caps leaking DC.

I can't really get into specifics like voltages you should expect as I've not been inside one of those yet.

Thanks. All plate voltages are fine and there's no leaking DC through the couple caps. No arcing evidence on the power tube socket either, so I'm guessing all that's left is the OT.

What's the easiest way to check it? As this amp was designed to take multiple power tubes I don't know what the turns ratio would be. Would checking the resistance of the windings with a DMM give any clues? It's a tiny little tranny - here's a pic of the guts:



I know what you're saying about a low wattage amp and an attenuator, but it was late and this thing is still very loud when dimed. I didn't realise they were prone to frying transformers though. THe speaker output on this amp is 4-16 Ohms, and it takes multiple power tubes from 6V6's to 6550's so I thought it might be pretty resiliant - I guess not!

I have emailed the builder to see if he can supply a replacement transformer - there's no markings on it, so I don't know if it's off the shelf or custom wound.

Thanks.

[Dodge]

Ok - I've measured up some resistances across the windings:

Primary (B+ to plate): 234 ohms
Secondary: 1.5 ohms

I guess that means there's an internal short on the secondary?

[MWJB]

It's the primary that would normally burn up...234ohms would seem quite healthy given the lack of more specific figures. Sometimes you can have an OT that reads good but arcs & shorts under load. However, I'm not sure your going about this the most effective way...OT is the last thing in the chain before the speaker, your problem could very easily be somewhere well before that in the circuit...if it is an attenuator related failure I'd be looking at the PT first.

Measure AC voltages to ground from the red wires going from PT to the rectifier socket, measure dc voltage at pin 8 of the rectifier.

Measure plate, screen & cathode dc voltages for the power tube (pins 3, 4, 8), plate & cathode voltages for the preamp tube (pins 1, 6, 3, 8) - read up on safe practices first & report back.

[Dodge]

Measure AC voltages to ground from the red wires going from PT to the rectifier socket, measure dc voltage at pin 8 of the rectifier.

Sorry - I had checked this already. With a 5Y3, a 6L6 and a 12AX7 installed

Red to red is 700 VAC (no tubes installed)

With a 5Y3, a 6L6 and a 12AX7 installed, DC at pin 8 on the rectifier is 378 V

Measure plate, screen & cathode dc voltages for the power tube (pins 3, 4, 8), plate & cathode voltages for the preamp tube (pins 1, 6, 3, 8) - read up on safe practices first & report back.

6L6 plate: 366 V
6L6 screeen: 357 V
12AX7 plates: 198 V
12AX7 cathodes: 1.5 V (V1a) 1.7 V (V1b)

There's no DC leakage though the coupling caps.

Thanks.

[MWJB]

Sorry, yes just re-read earlier posts...never mind didn't hurt to verify.

Next step if it's the OT is to check turns ratio, you'll need 2 meters & a voltage source (signal generator/variac) apply 0.5VAC to the speaker socket (OT sec) & measure voltage devolped at primaries of the OT.

Divide that voltage by 0.5, square it, then mutiply by speaker load.

E.g. 12.5VAC accross primary

12.5/0.5VAC = 25

25 squared = 625

625*8ohms = 5000

...if you had a 5K primary (sort of the region I'd be expecting).

Or, you might find it quicker & easier to just sub your OT for one with a 5K-ish primary and a healthy current rating on the primary (90mA?)

[MWJB]

Power tube cathode voltage? Just to be sure ;-)

[PRNDL]

Did you check the electrolytics (by measuring their resistance)?

[Dodge]

Just to close this thread - it was the OT, and it's now been replaced. The designer also sent me a higher value output tube cathode resistor to control the current which is now standard on his more recent models.

Many thanks for your help guys.